A new study combining temperature data with fossil records of large mammals that existed across Africa during the last 4 million years calls into question a long-held idea that periodic climate fluctuations were important drivers of evolutionary change in mammals, including human ancestors.
The study, which was published in the journal Proceedings of the National Academy of Sciences, provides an African continent-wide synthesis of environmental variability during the Plio-Pleistocene, a period in Earth’s history spanning roughly the last 5 million years and including the last ice age about 20,000 years ago.
Environmental variability throughout that time period, according to the study, reflects variations in the Earth’s orbit and orientation with respect to the sun, as predicted by Milankovic cycles, natural phenomena. These cycles subject our planet to variable levels of solar radiation, resulting in well-documented cyclical effects on Earth’s climate at different frequencies.
Due to changes in global ice volume and ocean temperature, the researchers discovered a long-term pattern of increased environmental variability across Africa. The findings, however, did not reveal a substantial link between environmental variation and rates of species origination or extinction, implying that environmental variability and species turnover are not necessarily linked, a theory that has sparked heated discussion among scientists.
According to Andrew Cohen, a University Distinguished Professor in the University of Arizona Departments of Geosciences and Ecology and Evolutionary Biology, the idea that long-term trends toward a wetter or drier environment may have been a driver of human evolution dates back to Charles Darwin’s time. In the late 1990s, a fundamental shift occurred when the important variability selection theory was introduced to the scientific community.
“The idea here is that it’s not just the direction of climate change that was important as a driver for evolutionary novelty in the hominin lineage, but the variability in the environmental and climate conditions,” Cohen explained.
“As our ancestors faced rapidly shifting conditions, this hypothesis suggests they had to be more resourceful and capable of dealing with many different contingencies, which, in turn, led to new species appearing while others went extinct.”
Researchers investigated sediment cores from lakebeds, ocean floors, and terrestrial outcrops from 17 places across the African continent and neighboring areas for the current study.
The idea here is that it’s not just the direction of climate change that was important as a driver for evolutionary novelty in the hominin lineage, but the variability in the environmental and climate conditions. As our ancestors faced rapidly shifting conditions, this hypothesis suggests they had to be more resourceful and capable of dealing with many different contingencies, which, in turn, led to new species appearing while others went extinct.
Andrew Cohen
Pollen, fossilized algae, dust, leaf waxes, soil isotopes, and other physical features that provide information about the types of plants and environmental conditions at the site where they were deposited were used to gather the environmental data.
Cohen said the team had to overcome a key obstacle in order to merge data from these quite different types of records and figure out the underlying pattern of climate variability: quantifying variability and comparing it from one sampling area to another.
“This isn’t trivial because you have records on the one hand of things like fossil pollen telling you about how variable the vegetation was, others telling you about changing lake levels, still others telling you about dust blowing out onto the ocean,” he said. “We needed a way to not just look at one record but stack all these different types of reference that allows us to tease apart the rhythm of variability.”
To do so, the researchers devised statistical tools that allowed them to “compare apples and oranges,” as Cohen put it, and grouped the climate datapoints into “bins” of 20,000, 100,000, and 400,000 years.
The team could then “stack” the different datasets of variability scores in each bin and calculate a mean amount of variability for each time period once they had been normalized.
The temperature data was then compared to the fossil record of large mammals from eastern Africa, especially bovids, a family that includes antelopes and other large herbivores. Because fossils of human predecessors are too scarce to be useful in such an approach, the researchers focused on giant herbivores.
“I won’t say you could fit all of (the hominin fossils) in a shoebox anymore, but they’re still not that common,” Cohen said, “so we decided to look at other organisms with a better fossil record, because there’s no reason to think that only our closest relatives, our hominin ancestors, should be affected by climate change and variability.”
“If climate variability is a significant driver in evolution, it ought to be a driver and evolution of other large mammals, too,” he added. “Think, for example, of polar bears and how they are affected by current climate change.”
The researchers utilized a strategy derived from modern wildlife population biology to account for a bias that has long plagued paleontologists: the fossil record’s inherent incompleteness, as illustrated by Andrew Du, the study’s second author. If a core sample was drilled through cheese, gaps would appear where the core impacted a hole in the cheese.
Similarly, a species’ fossil record comprises times when no fossils have been discovered mixed with periods when fossils have been discovered. This makes determining when a species first appeared in the fossil record and when it became extinct extremely challenging.
To get around this limitation, Du employed a procedure called catch, mark, and recapture, which is commonly used by wildlife scientists when conducting population surveys:
An animal is caught, tagged, and released back into the wild after being caught. Scientists compare the proportion of tagged and untagged animals in a later survey. They can gain an understanding of the size and organization of the population at large by using statistics.
Du, an assistant professor in Colorado State University’s Department of Anthropology and Geography, detailed how the technique works in fossil systems.
“Let’s say we see the appearance of a new species in the fossil record in time period one, then we find a different fossil from the same species in time period two, we miss it in time period three, but we see it again in time period four,” he said.
“What this tells us is that even though we didn’t see the species in time period three, we know it was around. This gives us an idea about the quality of the fossil record during certain time periods, and we can account for this quality when estimating speciation and extinction rates.”
The researchers were able to evaluate patterns of environmental variability and their relationship to mammal species origination and extinction rates after combining all of these datasets.
“Overall, there has been a long-term trend over the last 3.5 million years of increasing variability in the environment,” he said.
“That trend tracks rising variability in global ice volume and sea surface temperatures around Africa. Superimposed on that, we found another trend: Once we get into the ice ages, we see more ups and downs; the wiggles get bigger and bigger and bigger, reflecting the waxing and waning of the ice sheets, and that variability tracks the 400,000-year Milankovic cycles.”
Meanwhile, the fossil record of species origination and extinction in large herbivores, as well as hominid fossils, appears to be unrelated to these climate fluctuation trends.
While the authors acknowledge that the variability selection hypothesis may still be valid at different scales, they aim to inspire the scientific community to think about it more critically, “rather than just accepting it as an underlying principle of how we look at the fossil record in Africa, and especially the human fossil record,” Cohen said.
“We don’t say that environmental variability is not important for human evolution, but the data we have currently compiled is very inconsistent with that idea,” he said. “If environmental variability was as important as it has been made out to be, we would expect to see that long-term trend of increasing variability mirrored in evolutionary turnover in all kinds of species, including hominins, but we just don’t see that.”
